IOCAS-IR  > 海洋环流与波动重点实验室
中高纬海冰和背景流对内孤立波生成演化的调制机制
张沛文
Subtype博士
Thesis Advisor徐振华
2021-05-21
Degree Grantor中国科学院大学
Place of Conferral中国科学院海洋研究所
Degree Name理学博士
Keyword内孤立波 非静力数值模拟 生成过程 极地 背景剪切流
Abstract

海洋内波是发生在具有稳定密度层结的海水中的一种波动过程。内孤立波作为特殊的一种海洋内波,也被称作非线性内波。早期内孤立波的研究区域主要关注于中低纬度,目前已经形成了系统的认知。而对于高纬度地区内孤立波的生消过程的了解则仍显不足。在高纬度海域,海冰的存在是一个非常独特的环境因子,这一独有的环境特征使得内孤立波在极地的生成传播及耗散过程中呈现一些与中低纬度不同的机制与性质,因此研究极地内孤立波在海冰下的生成,传播及耗散过程对于推动高纬度内孤立波的理解认识十分重要。

低模态内孤立波是中低纬度内孤立波生消过程研究的主要对象,对于其破碎耗散过程的理解目前已经较为全面。近些年来,随着二模态内孤立波为代表的高模态内孤立波被越来越多的被观测到,二模态内孤立波存在时间较短,强耗散的特点也引起了注意。但目前对高模态内孤立波具体耗散机制的了解仍存在一定的不足,尤其是二模态内孤立波在背景剪切流中的耗散过程。因此重现二模态内孤立波在背景剪切流中的调制耗散过程,这对于揭示二模态内孤立波的局地湍流耗散过程,以及完善内孤立波理论体系都具有重要意义。

综上,本文通过结合非线性且非静力的数值模式及观测资料,针对高纬度区域及高模态内孤立波,开展了以下工作:

首先,回顾了中低纬度内孤立波的研究成果以及常用于内孤立波数值模拟的非线性,非静力MITgcm模式以及其在内孤立波相关工作中的应用情况。针对其模块化的结构进行了简单的阐释并介绍了几个对本文数值模式实验开展比较重要的模块的功能。

其后,模拟了高纬度海域正压潮流与冰脊相互作用,发现在这一过程中能够激发多模态内孤立波并伴随强混和过程。同时正压潮的能量也传递到了内孤立波中并向外传播。据此首次提出了在海冰覆盖的高纬度海区内孤立波的一种独特的生成机制,为研究冰脊下内孤立波的源区提供了新的思路。借助敏感性实验,证明了正压潮流强度和冰脊深度能够显著调制冰脊下内孤立波的生成过程。

再后,研究了内孤立波在冰脊下的传播耗散过程。结合冰脊的理论特征模型,发现在冰脊的调制作用下,内孤立波会在冰脊附近裂变成多个次级波并伴随着能量向次级波中的转移过程。当内孤立波在冰脊附近裂变时,还会显著强化冰脊附近的湍流混合与垂向热量输送,这对于冰脊的生成和融化会产生一定的影响。敏感性实验表明冰脊的深度和宽度会显著调制次级波的生成,改变内孤立波在冰脊附近的能量分配过程,同时会直接影响冰脊附近的湍流混合强化的程度。

最后,探讨了二模态内孤立波在背景剪切流中的耗散过程。研究了二模态内孤立波在背景剪切流中的调制裂变过程与机制。背景剪切流的存在会激发剪切不稳定,进而破坏二模态内孤立波的结构并引起内孤立波的失稳裂变,并生成内呼吸子等特殊结构。实验证实了背景剪切流的存在是造成二模态内孤立波的短暂存在及强耗散现象不可忽视的原因。背景剪切流的强度,相对于跃层的偏移程度等剪切流特征对于二模态内孤立波的裂变和耗散过程也有显著影响。

内孤立波作为一种强非线性动力过程,因其具有物质输运,跨尺度能量传递以及调节垂向热平衡的能力而备受关注。本文的亮点在于首次提出了一种海冰覆盖区正压潮流和冰脊相互作用激发内孤立波的生成机制,并以此为基础,结合观测和冰脊形态的统计模型揭示了内孤立波在冰下传播过程中受到冰脊调制并强化局地混合的过程,同时量化估计了这一过程中的热效应,展示了内孤立波在冰脊下的生成,传播,演化及破碎过程对于冰下垂向热输送及热平衡的影响不可忽视。同时,聚焦二模态内孤立波的短暂存在现象,证实了背景剪切流是造成二模态内孤立波强耗散的一个重要因素,并且二模态内孤立波的变形裂变以及耗散强度和背景剪切流的特征密切相关。这对于解释二模态内孤立波对于局地混合贡献十分重要。因此,本文的工作不仅拓展了高纬度内孤立波的生成传播机制的研究,也对于中纬度高模态内孤立波的耗散机制进行了补充和完善。同时,这对于海洋生态资源开发、海洋环境保护、高纬度海域水下航行安全以及海上工程的实施都有着应用价值。

Other Abstract

Internal waves are a kind of fluctuating process that occurs in stratified oceans. As a special kind of internal waves, internal solitary waves (ISWs) are also called nonlinear internal waves. The research of ​​early ISWs mainly focused on low and middle latitudes regions, and a systematic understanding has been formed. However, the understanding of the generation and dissipation process of ISWs in high latitudes is still remain insufficient. In high-latitude seas, the existence of sea ice is a very unique environmental factor. This unique environmental feature makes the generation, propagation and dissipation of ISWs in the Polar Regions demonstrate some mechanisms and properties that are different from those of middle and low latitudes. Investigating the generation, propagation and dissipation processes of ISWs in Polar Regions under sea ice is very important to promote the understanding of ISWs in high latitudes.

Low-mode ISWs are the main object of previous studies on the generation and dissipation of ISWs in low and mid-latitudes, and the understanding of their breaking and dissipation processes is now relatively comprehensive. In recent years, as high-mode ISWs represented by mode-2 ISWs have been observed more frequently, the ephemeral existence and highly dissipated nature of mode-2 ISWs draw much attention. But the current understanding of the specific dissipation mechanism of high-mode ISWs is still insufficient, especially the dissipation process of mode-2 ISWs in the background shear flow. Therefore, reproducing the modulation and dissipation process of the mode-2 ISWs in the background shear flow is of great significance for revealing the local turbulent dissipation process of the mode-2 ISWs and improving the theoretical system of ISWs.

In summary, this paper combines nonlinear and non-hydrostatic numerical models and observations to focus on high-latitude regions and the high-mode ISWs, the following work has been carried out:

Firstly, it reviews the research results of ISWs in middle and low latitudes, as well as the nonlinear, non-hydrostatic MITgcm model commonly used in the numerical simulation of ISWs and its application in ISW related work. It briefly explained its modular structure and introduced the functions of several modules that are important for the numerical simulations in this paper.

After that, the interaction of barotropic currents and ice keels in high-latitude seas was simulated, and it was found that multi-modal ISWs can be excited in this process, accompanied by a strong mixing process. At the same time, the energy of the barotropic tides is also transferred to the ISWs and radiated outward. Based on this, a unique generation mechanism of ISWs in high-latitude sea areas covered by sea ice is proposed for the first time, which provides a new idea for studying the source of ​​ISWs under ice keels. With the sensitivity experiments, it is proved that the amplitude of barotropic tidal current and the depth of ice keel can significantly modulate the generation process of ISWs under ice keel.

Then, the propagation and dissipation process of ISWs under ice keels was studied. Combined with the theoretical characteristic model of the ice keel, it is found that under the modulation of the ice keel, the ISW will disintegrate into multiple secondary waves near the ice keel, accompanied by the process of energy cascade to the secondary waves. When the ISW disintegrate near the ice keel, it will also significantly strengthen the turbulent mixing and vertical heat transfer near the ice keel, which will have a certain impact on the growing and melting of the ice keel. Sensitivity experiments show that the depth and width of the ice keel will significantly modulate the generation of secondary waves, change the energy distribution process of ISWs near the ice keel, and directly affect the strength of turbulent mixing near the ice keel.

Finally, the dissipation process of the mode-2 ISWs in the background shear flow is discussed. The modulated and disintegrated mechanism of the mode-2 ISWs in the background shear flow are studied. The existence of background shear flow will stimulate shear instability, which will destroy the structure of the mode-2 ISWs and cause the destabilization and disintegration of the ISW, and generate special structures such as internal breathers. Experiments have confirmed that the existence of background shear flow is the reason for the ephemeral existence and highly dissipated nature of mode-2 ISWs that cannot be ignored. The amplitude of the background shear flow and the degree of offset relative to the pycnocline of the background shear flow can also make significant impact on the disintegration and dissipation of the mode-2 ISWs.

As a strong nonlinear dynamic process, ISWs have attracted much attention because of their ability to transport materials, transfer energy across scales, and adjust vertical heat balance. The highlights of this paper is that for the first time, a mechanism for generating ISWs triggered by the interaction of barotropic tides and ice keels beneath sea ice covered areas is proposed. Based on this, combined with observations and statistical models of ice keel revealed propagation of ISWs is modulated by the ice keel and the process can enhanced local mixing. At the same time, the thermal effect in this process is quantitatively estimated, showing that the generation, propagation, evolution and breaking of ISWs under the ice keel have an impact on the vertical heat transport and heat balance. Noted that this influence cannot be ignored.

On the other hand, this paper focused on the ephemeral existence of the mode-2 ISWs in the background shear currents. The results confirmed that the background shear current is an important factor causing the deformation, disintegration and strong dissipation of the mode-2 ISWs. It is closely related to the characteristics of background shear current. This is very important for explaining the contribution of mode-2 ISWs to local mixing.

Therefore, the work of this paper not only expanded the research on the generation and propagation mechanism of ISWs in high latitudes, but also supplemented and improved the dissipation mechanism of high mode ISWs in mid latitudes. In the meantime, it has application value for the exploiting of marine ecological resources, marine environmental protection, and the safety of high latitude underwater navigation as well as deployment of offshore engineering.

Subject Area地球科学 ; 海洋科学
MOST Discipline Catalogue理学 ; 理学::海洋科学
Language中文
Document Type学位论文
Identifierhttp://ir.qdio.ac.cn/handle/337002/170701
Collection海洋环流与波动重点实验室
Recommended Citation
GB/T 7714
张沛文. 中高纬海冰和背景流对内孤立波生成演化的调制机制[D]. 中国科学院海洋研究所. 中国科学院大学,2021.
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